Conventionally, a variety of methods has been used as bolt-axial-force measuring methods for measuring the axial forces of the bolts, which are screw-joined to the nuts and fasten the fastened members.
A load cell method is a method in which: a load cell is sandwiched between an intermediate of a bolt to be fastened or a bolt and a fastened member; under that state, the bolt is fastened to compress the load cell; and a compressive force to be detected by the load cell is directly detected as an axial force of the bolt.
A strain gauge method is a method in which: a strain gauge (strain meter) is pasted on an axial peripheral part of the bolt; by a means of this strain gauge, the minimal axial-directional strain (elongation) generated in fastening the bolt is detected; and the bolt axial force is calculated based on this strain.
An ultrasonic method is a method in which: by using an ultrasonic oscillator and receiver, axial-directional length dimensions of the before-fastened and after-fastened bolts are measured precisely; and the bolt axial force is calculated based on elongations of the before-fastened and after-fastened bolts acquired from results of the measurements. For example, disclosed in Japanese Patent 1 (Japanese Patent Application Laid-Open Publication No. 2006-308342) is a method in which: an ultrasonic oscillator and receiver are placed on one axial-directional end surface of a before-fastened bolt; an ultrasonic wave is caused to be incident from a one end surface of the bolt; the axial-directional length of the before-fastened bolt is measured based on a time required until the reflected wave of the ultrasonic wave from the other axial-directional end surface of the bolt is received and, at the same time, the axial-directional length dimension of the after-fastened bolt is measured in the same manner after the bolt is fastened; and the bolt axial force is calculated based on the bolt elongations calculated from the results of these measurements.
A measuring stick method is a method in which: a measuring stick is inserted into a deep hole formed inside an axial center of the bolt; an amount of projection of the measuring stick from the deep hole before and after the fastening of the bolt is measured by a means of a length measuring instrument such as a dial gauge; and the bolt axial force is calculated based on the bolt elongation acquired from this measurement.
However, since the load cell method or the strain gauge method necessitate placing the load cell or pasting the strain gauge on each bolt, it is possible to use the methods when the axial forces of a small number of bolts are experimentally measured. However, when the axial forces of all of a large number of bolts provided in or on an apparatus and the like must be measured, since an installation operation of the apparatus and a measuring operation itself become complicated, it is difficult to adopt these methods.
In the ultrasonic method, a small type measuring instrument for measurement has been developed. However, in the ultrasonic method, there are many factors affecting measurement accuracy, such as a degree of parallelism of both axial-directional end surfaces of the bolt, finish surface roughness, uncertainty regarding replication at measurement locations of contacts of the ultrasonic oscillator and receiver, a difference between transmission rates of the ultrasonic waves due to quality of a material of the bolt, roughness and fineness of a crystal of the quality of the material of the bolt, a temperature at a measuring time (a change in the transmission rates of the ultrasonic waves), an influence of stress distribution inside the bolt at a time of being fastened, and response linearity and sensitivity adjustment of the ultrasonic oscillator and receiver. Hence, each measuring operation requires a certain degree of skill, and it is difficult to obtain the stably measured results with high accuracy. Further, although the measuring instrument having a function to correct the influences of those factors based on theories and experiences of each maker itself is also developed, since their basis is not revealed, there is no alternative but to believe data provided from the measuring method designated by the maker.
A method using the measuring stick needs to drill deep holes for all of the bolts as measuring targets. The drilling of the deep holes has a problem in that since it requires not a usual drilling machine but a drilling machine equipped with a special gun drill, a drilling cost becomes high. Further, in the method using the measuring stick, it is extremely difficult to obtain, for the following reason, theoretically a relationship between a standard stick length, which serves as a standard for measuring an axial-directional elongation value of the bolt, and a fastening effective length, which is necessary for conversion calculation into the bolt axial force, so that at present the elongation of the bolt is converted into the axial force using an empirical value and/or an approximately related formula. Namely, it is difficult to specify the actual axial force of the bolt because of mixed stresses per cross-sectional areas which are generated along with the above axial force at the respective cross-sectional shapes of a screw part, an axis part, and the other reaction force side screw part (or a bolt head part) of the bolt being different from one another, i.e., variation in each length direction of those parts. In addition, compressive stress distribution of a nut and the reaction force side seat surface, a shape of a flange to be fastened, elastic coefficient variation as a compressed-side system affected by a nipped gasket, and a partial deformation due to inclination of the screw itself that is engaged with the nut in an axial direction of the screw part also change depending on a degree of the fastening axial force. As a result, a distribution state of the amount of the axial-directional elongation of the bolt varies depending on a change of the axial force, and is extremely complex. Accordingly, it is extremely difficult to theoretically quantitatively evaluate a definite relationship between the measuring stick serving as a standard and the effective fastening length. Therefore, it is the usual common practice of controlling the estimated bolt fastening force based on the bolt elongation value that is calculated on the estimation of the bolt effective length generally by using an empirical value and a numerical value equivalent to a thickness of the fastened flange plus a bolt screw diameter, or its multiplied value by a safety factor.